1. Field of the Invention
The present invention relates to an power conversion device that converts DC power into AC power, or AC power into DC power.
2. Description of Related Art
A power conversion device that receives DC power and converts that DC power into AC power for supply to a rotating electrical machine incorporates a plurality of switching elements. The DC power that is supplied is converted into AC power by these switching elements repeatedly performing switching operation. Many such power conversion devices are also used for converting AC power that is generated by a rotating electrical machine into DC power by the switching operation of the above described switching elements. It is per se known to control the switching elements described above on the basis of a pulse width modulation method (hereinafter termed the “PWM method”) that uses a carrier wave that varies at a constant frequency. By increasing the frequency of the carrier wave, the accuracy of control may be enhanced, and moreover there is an accompanying tendency for the torque generated by the rotating electrical machine to become smoother.
However, the power losses when the switching elements described above are changed over from their discontinuous states to their continuous states and from their continuous states to their discontinuous states become greater, and the amount of heat generated also becomes greater.
An example of such a power conversion device is disclosed in Japanese Laid-Open Patent Publication No. S63-234878.
It is desirable to reduce the power losses described above caused by the switching elements, and, by reducing the power losses, it is also possible to reduce the amount of heat generated by the switching elements. For this, it is desirable to reduce the number of times that the switching elements described above are switched. However, as described above, with a per se conventional PWM method, if the frequency of the carrier wave is reduced in order to reduce the number of times that the switching elements described above perform their switching operation per unit time, the distortion of the AC current outputted from the power conversion device becomes greater, and this is also accompanied by increase of torque pulsations.
One object of the present invention is to provide a power conversion device with which it is possible to anticipate reduction of the switching losses, or to provide a control method for a power conversion device with which it is possible to anticipate reduction of the switching losses
In addition to solving the problems described above, the power conversion devices according to the embodiments described below can also solve the problem of maintaining high reliability, even when the state of the electric load has changed.
The embodiments explained hereinafter reflect the results of much desirable research for production of this power conversion device as a manufactured product, and solve various concrete problems that need to be solved for production as a manufactured product. Some such concrete problems that are solved by the concrete structure and operation of the embodiments described below will be explained hereinafter in connection with the description of those embodiments.